1. Field of the Invention
The present invention relates to an image forming optical system (zoom optical system) that has a high zoom ratio and good image forming performance while being slim and is particularly suitable for use as an electronic image pickup optical system, and to an electronic image pickup apparatus equipped with such an image forming optical system.
2. Description of the Related Art
Digital cameras have reached practical levels in terms of large number of pixels (high image quality), compactness, and slimness and replaced 35 mm film cameras from the view points of functions and market. As one aspect of further evolution, a further increase in the number of pixels is strongly desired to be achieved along with an increase in the zoom ratio and an increase in the angle of view while keeping the smallness and slimness as they are.
Zoom optical systems that have been used for their advantage in achieving high zoom ratios include, for example, an optical system disclosed in Patent Document 1. Patent Document 1 discloses what is called a positive-front zoom optical system including, in order from its object side, a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, a fourth lens group having a negative refracting power, and a fifth lens group having a positive refracting power.
This optical system has high image forming performance while having a zoom ratio of 5 to 10 and an F-number at the wide angle end of 2.4. However, the thickness of each lens group along the optical axis direction is large. In consequence, it is difficult to make the camera body thin even if a collapsible lens barrel is used. The collapsible lens barrel refers to a lens barrel of a type in which the lens barrel unit is housed in the camera body along the thickness (or depth) direction.
Patent Document 2 discloses an optical system that is designed to be thin with thinned lens elements and a reduced number of lens components while achieving excellent correction of chromatic aberration. To achieve such an optical system, Patent Document 2 teaches to use a transparent medium having effective dispersion characteristics or partial dispersion characteristics that conventional glasses do not have. However, the shape and arrangement of the lens made of the aforementioned transparent medium is not necessarily appropriate, and it cannot be said that a sufficient reduction of the thickness of the optical system can be achieved.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-255228    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-145823